1. Field of the Invention
The present invention relates to a DC power source unit that can be used for cordless electrically powered tools.
2. Description of the Related Art
Cordless electrically powered tools, such as electrical drills, can be used any place and are easy to use because no power source cables are required. However, the use of batteries invites some disadvantages. When the charge remaining in the batteries drops to less than an allowable limit, the batteries need to be replaced or recharged. Further, because the battery voltage gradually drops as the use of the battery progresses, the tool's operation efficiency is not constant.
FIGS. 1 and 2 show a conventional DC power source unit suitable only for situations wherein the tool is used close to an AC power source and when work with the tool can be accomplished in a single location without the need to move the tool to different areas. The DC power source unit includes an AC/DC converting box 50, an input cable 57, and an output cable 54. The input cable 57 has one end connected to the converting box 50 and another end attached to a plug 56. The plug 56 is used for connection to an AC power source. The output cable 54 has one end connected to the converting box 50 and another end attached to an adapter plug 55. The adapter plug 55 is detachably connected to the electrically powered tool. As shown in FIG. 2, the converting box 50 includes a transformer 51 for dropping the AC 100 volts to ten and several volts, a rectifier 52 for rectifying the AC voltage to a DC voltage, and a smoothing capacitor 53 for smoothing the waveform of the rectified voltage.
With the DC power source unit shown in FIGS. 1 and 2, the AC voltage supplied from a commercial outlet is converted to a DC voltage to be supplied to the tool. Such a DC power source unit allows the continuous use of tools, thereby ensuring constant work efficiency.
The motors contained in electrically powered tools do not heated up significantly when the tool is driven with batteries. For this reason, the battery powered tools are, generally, not provided with motor cooling fans. Even if the fans are provided, they are small size fans.
When a tool is driven using the conventional DC power source unit, the tool tends to be used continuously for a long period of time, causing the temperature of the motor to rise significantly. This shortens the service life of the motor or the tool. To overcome the above problem, one solution would be providing a large-size cooling fan within the electrically powered tool. However, a large-size cooling fan would increase the overall size of the tool. Large size tools are not desirable for the operator to handle or operate. Another solution to this problem would be providing a temperature sensor in the tool to monitor an abnormal temperature rise within the tool. Power supply to the motor can be controlled in response to the output of the sensor. However, it is difficult to add a new component to the electrical circuitry of the tool and to modify control circuitry of the tool. Even if such addition and modification of the control circuit were made, the resultant electrical arrangement would be complicated.